Sport shoes for track and field competitions, in particular for sprinting over short distances, have conflicting requirements. For example, the shoes should be as lightweight as possible, because the weight of the shoes can obstruct fast movements of an athlete during the sprint. The importance of lightweight construction follows from the fact that a reduction of the weight of the shoe by 30 g leads to a reduction of energy consumption during running of 0.3%; however, the shoes must have sufficient stability against deformation so that the foot is sufficiently supported and guided during running.
Another consideration in the construction of sprint shoes is the elastic storing of energy by the shoe during the course of movement. During each landing phase, the shoe is deformed in the forefoot part by the rolling-off with the ball of the foot and the toes. During the subsequent push-off with the toes, the foot is straightened and the shoe returns to its original shape. These movements are repeated with each step during running.
In contrast to the commonly used layer ensemble of foamed materials for the forefoot part of a normal sports shoe, it may be possible to provide for an elastic storing of energy needed for the deformation of the shoe by a flat bending elastic plate in the forefoot part of sprint shoes, which extends into the mid and rearfoot part. In a step cycle, this plate is bent in its longitudinal direction during the rolling-off phase and elastically springs back during the subsequent push-off to its original shape and thereby supports the course of movements of the sprinter.
One example of such a bending elastic plate is disclosed in U.S. Pat. No. 5,052,130. The essentially flat plate of carbon fibers disclosed therein has a great bending stiffness in a longitudinal direction. It occupies the complete width of the sole in the forefoot part, but is considerably narrower in the rearfoot part, purportedly to allow, apart from storing energy, good damping of the shoe by viscous materials during first ground contact.
Another example of a bending elastic plate is disclosed in European Patent No. 0 272 082. Here, the flat plate also extends essentially over the complete length of the shoe. An additional damping material may be provided in the rearfoot part, purportedly to reduce the stress on the foot during the ground contact of the heel.
Sprint shoes according to the above discussed prior art however, have the disadvantage that the spring force of the elastic plate is not sufficiently transmitted during push-off to the complete foot. In particular, the heel part is not sufficiently included in the overall procedure due to the softer materials provided in the heel part. Although the plate itself stores energy elastically, with minimal losses, the return of invested energy, the intended effect, is only partially achieved.